Fuel pumps have so far been widely used in internal combustion engines of automobiles, among others. When brushes slidingly come into contact with a plurality of divided contacting portions of the commutator in the motor section, electric current flows from a power supply source to the armature windings, and the armature turns. As a result of the rotation of this armature, the impeller in the pump section rotates, and the fuel is sucked up from the fuel tank and fed to an internal combustion engine.
The commutator is generally made of copper. When the brushes, which slidingly come into the copper-made contacting portions, are soft, the brushes will wear quickly, hence the life thereof will be shortened. For example, it is conceivable to form the brushes out of a carbon material comprising amorphous carbon, which is high in hardness, to thereby improve the wear resistance of the brushes. However, the copper-made contacting portions may be corroded as a result of their reaction with an oxidized fuel or sulfur component-containing fuel, for instance. Further, copper sulfide, which is electrically conductive, may be formed, possibly resulting in electric connection between two neighboring contacting portions out of the plurality of divided contacting portions. For preventing the contacting portions from reacting with the fuel, it is known in the art that the contacting portions be made of a carbon material, as disclosed in U.S. Pat. No. 5,175,463.
However, the contacting portions made of a carbon material are low in hardness and inferior in mechanical strength as compared with contacting portions made of copper, so that there arise problems, namely when the contacting portions made of a carbon material slidingly contact with the brushes made of an amorphous carbon-containing material, the contacting portions are worn at an increased rate and the life of the contacting portions until reaching the tolerance limit of wear becomes shortened. An attempt has been made to prolong the life of contacting portions by using artificial graphite, which is higher in hardness than natural graphite, as the carbon material for making contacting portions. However, there arises another problem, namely the production cost increases since artificial graphite is expensive as compared with natural graphite.
Thus, it has been disclosed in Japanese Patent Laid-Open Application (JP Kokai) H10(1998)-162923 to add 5 to 30% by weight of amorphous carbon to natural graphite.
However, when natural graphite is used as the key material, the life of commutators is limited. Further, in view of the increasing trend in recent years toward cost reduction, it is difficult to manufacture commutators having satisfactory characteristics using natural graphite as the key material while meeting the cost reduction requirement.
Accordingly, it is an object of the present invention to provide a commutator which is excellent in wear characteristics and can be manufactured at low cost and can be used in fuel pumps.